Carbon black is used in several applications, but is primarily used as a reinforcer for elastomeric compositions such as the rubber in tires. Carbon black which is appropriate for reinforcing rubber often has a very low bulk density. Low bulk density carbon black occupies a relatively large volume and is therefore difficult to ship in bulk form and handle when mixing with rubber. Accordingly, carbon black is beaded or pelletized to increase the bulk density of the carbon black and render the carbon black more readily handleable in bulk form.
One common method of beading carbon black is by feeding carbon black powder through a cylinder containing a rapidly rotating pin shaft. A liquid beading additive is normally added to the carbon black to promote the beading process and impart good handling characteristics to the carbon black powder as the powder passes through the beading apparatus. Normally the beaded carbon black is then dried in a rotary drier.
Most conventional bead additives are materials which carbonize during the drying of the carbon black beads. These conventional bead additives include substances such as lignosulfonates, molasses, sugars, and the like and are normally added to carbon blacks in a water solution. The carbonized bead additives tend to hinder the dispersion of carbon blacks in rubber compounds. Thus, the costs saved by the improved handling characteristics imparted by most conventional bead additives are somewhat offset by the additional energy costs necessary to effectively disperse the carbon blacks treated with conventional bead additives. Further, the carbonized bead additives tend to hinder the reinforcement properties of the treated carbon blacks in rubber.
Even after carbon black is beaded with conventional bead additives, fine carbon black agglomerates, called fines, may be generated from carbon black beads and become airborne during handling of the carbon black. This effect is often called dusting. Efforts have been made to solve this particular problem. U.S. Pat. No. 4,440,807 to Gunnell proposes a method for covering carbon black beads with a layer of rubber to prevent dusting. The carbon black bead disclosed in Gunnell is described as consisting essentially of a carbon black pellet core and a cover layer of rubber. In other words, the carbon black pellet or bead disclosed in Gunnell has a core of carbon black which is not coated with elastomer. Accordingly, although the carbon black bead disclosed in Gunnell may exhibit less dusting during handling, the core of the carbon black bead in Gunnell is the same as a conventional carbon black bead.
U.S. Pat. No. 4,569,834 to West et al. discloses a process for beading carbon black with polyalkene bead binder to improve the dispersion of the beads in rubber and other polymers such as polyethylene. Preferred pelletizing additives disclosed in West are substituted or unsubstituted polyalkenes such as surface-oxidized polyethylene. West specifically discloses the use of AC-69 polyethylene from Allied Chemical as a carbon black bead binder. AC-69 is a low-molecular weight material that is not a high polymer or an elastomer. AC-69 polyethylene is a waxy material that is substantially saturated and, as such, may be expected to give some benefits in mixing because of a lubricity effect, but such polyethylenes do not chemically interact with elastomers. In rubber tires containing carbon black, the chemical interaction between the carbon black and elastomer reinforces the elastomer and improves the performance of the tire. Thus, when mixed with elastomer as in a rubber tire, carbon black beads disclosed in West et al. can block reinforcement of the elastomer by the carbon black.
U.S. Pat. No. 2,457,962 to Whaley discloses methods of beading carbon black with latex as a wetting agent to improve the handling properties of the carbon black and dispersion of the carbon black in rubber. In one method disclosed in Whaley, carbon black beads are formed by wetting the carbon black with latex and then agitating the mixture in a conventional carbon black pelletizer. Whaley teaches that the rubber content of the latex beaded carbon black should range from five to twenty percent by weight of the beaded carbon black. Although the beaded carbon black in Whaley may be effective in improving the handling properties of the carbon black and the dispersion of the carbon black in elastomer, the relatively large proportions of rubber that Whaley teaches should be incorporated into the carbon black significantly increases the cost in producing the beaded carbon black. This increased cost is a substantial disadvantage in the production of beaded carbon black which is to be used to make tires because such beaded carbon black typically has a very low profit margin. In addition, and perhaps more importantly, commercially available latex has significant quantities of surfactants that end up in the beaded carbon black product and in the final products, such as automobile tires, into which the beaded carbon black is mixed. In sufficient quantities, such surfactants can impair the ability of the carbon black to reinforce rubber and can otherwise impart undesirable properties to rubber.
Accordingly, there is a need for an economical beaded carbon black that is readily handleable in bulk form, disperses well in rubber, and effectively reinforces rubber.